Gas turbine apparatus and method of operating same

Abstract: In a gas turbine apparatus (113) including a turbine stage (123) and a compressor stage (125), when an abrupt load variation occurs in a centrifugal compressor (101) which increases pressures of a fuel such as city gas or liquefied natural gas to feed the fuel, a gas turbine control device (1) effects a control so that a discharge pressure of the centrifugal compressor can be kept to within a predetermined pressure range. When a fuel flow demand for the centrifugal compressor is inputted to the gas turbine control device, a signal corresponding to this fuel control signal is inputted to a bypass valve (107) of the centrifugal compressor simultaneously when it is inputted to gas turbine fuel control valves (42, 48) provided in the gas turbine apparatus. When the load is abruptly varied, the degree of opening of the bypass valve of the centrifugal compressor is changed to a value determined by a prestored relation between the fuel control signal and the degree of opening of the bypass valve.

Description: The invention relates to a gas turbine apparatus comprising a gas turbine with a turbine stage, a combustor, a turbo compressor for feeding city gas or liquefied natural gas under pressure to said combustor, and a gas turbine control device for controlling said gas turbine, which gas turbine control device comprises fuel flow rate control means for controlling the rate of the flow of combustible gas, and furthermore it relates to a method of operating a gas turbine apparatus using as a fuel liquefied natural gas or city gas fed under pressure by a turbo compressor. A gas turbine apparatus has an operational mode called "load shutdown" in which in the event of abnormality in a generator serving as a load, the load is disconnected to thereby protect the gas turbine. In this mode, the load of the gas turbine is abruptly changed from 100% to 0%, and this has heretofore been achieved by abruptly closing a fuel flow control valve. At this time, a rotational speed of the gas turbine is controlled by a gas turbine control device to be within a certain range. In the type of gas turbine apparatus using city gas or liquefied natural gas as a fuel, a centrifugal compressor is used for supplying the fuel to a combustor of the gas turbine apparatus. In the event of the above load shutdown, the rotational speed of this centrifugal compressor transiently increases, or a discharge pressure thereof increases. When this discharge pressure exceeds an allowable upper limit value, the fuel such as city gas or liquefied natural gas excessively flows into the combustor, so that the gas turbine may rotate at an unduly high speed. In order to prevent such a situation, there has heretofore been used a method in which the flow rate of the gas, handled by the centrifugal compressor for feeding the fuel under pressure, is measured, and when the flow rate decreases, a valve (hereinafter often referred to as "bypass valve") provided in a flow passage is opened. A gas turbine apparatus using such a bypass valve in a compressor recycle loop is known from the paper "Protecting Multicase Compressors" by N. Staroselsky and D. Carter, published in Hydrocarbon Processing, 69 (1990) March, No. 3, Houston, Texas, pages 85 to 89. By the way, when the flow rate of the handled gas is reduced, the pressure and the flow rate suddenly fluctuate in the centrifugal compressor to cause a surging phenomenon in which vibrations increase. In order to prevent this phenomenon, there is widely known a method as described above, in which the flow rate of the gas is measured, and if the flow rate is lower than the predetermined value, the rate of flow through the centrifugal compressor is increased to prevent the surging of the centrifugal compressor, and also the bypass valve or a discharge valve is opened, thereby controlling the rate of flow to be fed to the gas turbine to a predetermined level. Such surging prevention method is disclosed, for example, in "JSME Mechanical Engineering is Handbook, Revised 6th Edition" compiled by the Japan Society of Mechanical Engineers (1985; Vol. 10, pages 10-33). The above conventional control method, in which the rate of flow of the handled gas is measured, and when this flow rate decreases, the bypass valve is opened, suffers from a drawback that since the bypass valve used for controlling the flow rate of the fuel of the gas turbine is closed instantaneously (about 0.1 second), the discharge pressure of the centrifugal compressor exceeds the above upper limit value. A 100% load shutdown mode among operation modes of a gas turbine is one in which a load variation is the largest. In this operation mode, a signal (FFD: Fuel Flow Demand) for controlling the flow rate of the fuel of the gas turbine is instantaneously changed from 100% to 0%, so that a flow rate control valve is switched from an open condition to a closed condition, thus switching the gas turbine from a 100% load condition to a zero load condition. At the time of this load shutdown, the fuel flow rate control valve is switched from the open condition to the closed condition in about 0.1-0.2 seconds. Therefore, the response is too late with a conventional method of controlling a centrifugal compressor, which feeds the fuel gas, by a bypass valve, in which method the flow rate handled by the centrifugal compressor is monitored, and when the flow rate becomes less than a lower limit value, the bypass valve is opened. As a result, a discharge pressure of the centrifugal compressor increases instantaneously, so that the number of revolutions of the gas turbine exceeds an upper limit value. In the gas turbine apparatus, even at the time of the load shutdown, the discharge pressure of the centrifugal compressor is required to be within a predetermined range. To meet this requirement, there are proposed two methods, and one is to provide a tank having a large volume between the centrifugal compressor and the gas turbine so as to reduce a pressure fluctuation, and the other is to quicken the response to the control command of the centrifugal compressor. A gas turbine apparatus known from EP-A-0 377 292 and comprising a gas turbine with a turbine stage, a combustor, and centrifugal compressor for feeding city gas or liquefied natural gas under pressure to said combustor is directed to the latter method. Although the gas turbine apparatus according to EP-A-0 377 292 does not include a bypass valve, it includes a sophisticated gas turbine control device for controlling said gas turbine, which gas turbine control device comprises fuel flow rate control means for controlling the rate of the flow of combustible gas. A compressor flow control is implemented as a variable gain changer to a normal fuel gas flow control system. A gas control valve is always operated under authority of speed control and temperature control. It is an object of this invention to provide a gas turbine apparatus in which even when upon a load shutdown (load is abruptly varied), a discharge pressure of a centrifugal compressor or other turbo compressor for feeding a fuel under pressure to a combustor is prevented from increasing unduly, and also a rotational speed of a gas turbine is prevented form increasing unduly, thereby protecting the gas turbine apparatus, thus enhancing its reliability and to provide a method of operating such a gas turbine apparatus. According to the invention this object is achieved by a gas turbine apparatus comprising a gas turbine with a turbine stage, a combustor, a turbo compressor for feeding city gas or liquefied natural gas under pressure to said combustor, and a gas turbine control device for controlling said gas turbine, which gas turbine control device comprises fuel flow rate control means for controlling the rate of the flow of combustible gas, whereby said gas turbine comprises a bypass passage extending from an outlet of said turbo compressor to an inlet of said turbo compressor, through which bypass passage the combustible gas is able to flow from said outlet of said turbo compressor to said inlet of said turbo compressor, and an opening/closing means for opening and closing said bypass passage and said gas turbine control device comprises means for producing a signal for driving said opening/closing means and said fuel flow rate control means in synchronism with each other. The object is further achieved by a method of operating a gas turbine apparatus using as a fuel liquefied natural gas or city gas fed under pressure by a turbo compressor, comprising the steps of producing a fuel control signal in accordance with at least one of signals outputted respectively from start-up control means, acceleration control means, speed/load control means, load limitation means, and exhaust gas temperature control means provided in a gas turbine control device, and controlling a rate of flow of the fuel into said gas turbine apparatus as well as a rate of flow of the fuel around said turbo compressor by said gas turbine control device in accordance with said fuel control signal. According to the invention, the fuel flow demand (FFD) of the gas turbine is inputted early to the bypass valve control device of the fuel gas compressor in preparation for the control of the bypass valve, thereby quickening the response, so that even in the event of load shutdown, the discharge pressure of the turbo compressor is kept to within a predetermined range. It is preferred that said gas turbine control device comprises a storage means which beforehand stores a relation between a value of said signal produced by said signal producing means and a degree of opening of said opening/closing means. In another preferred embodiment of the gas turbine apparatus according to the invention said gas turbine control device comprises start-up control means, acceleration control means, speed/load control means, load limitation means, and exhaust gas temperature control means, and said means for producing the signal for driving said opening/closing means and said fuel flow rate control means in synchronism with each other is a selection means for selecting the least one among control signals outputted respectively from the above five control means and for outputting said least signal as said signal for driving said opening/closing means and said fuel flow rate control means in synchronism with each other. In this embodiment of the invention, even when an abrupt load variation due to load shutdown is encountered in the gas turbine apparatus, the fuel control signal, which is the output signal of the minimum signal selection circuit, is outputted also to the bypass valve of the turbo compressor, and therefore the bypass valve can be opened at an early timing, and the discharge pressure of the turbo compressor can be limited to a level below the upper limit value. As a result, the gas turbine can be operated within the range of the required rotational speed, so that the reliability of the gas turbine apparatus is enhanced. Furthermore, by reducing the degree of fluctuation of the discharge pressure of the turbo compressor, the volume of the discharge tank provided between the turbo compressor and the gas turbine can be reduced. In a preferred method according to the invention said fuel control signal is outputted to fuel flow rate control means of said gas turbine apparatus and flow-around flow rate control means of said turbo compressor in a synchronous manner. A preferred embodiment of the gas turbine apparatus according to the invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a schematic control flow diagram of one preferred embodiment of a gas turbine apparatus of the present invention; Fig. 2 is a control flow diagram of a main portion of the gas turbine apparatus of Fig. 1; Fig. 3 is a control block diagram of one example of a centrifugal compressor of the present invention; Fig. 4 is a diagram showing the relation between a valve opening degree and a fuel flow; Fig. 5 is a time chart at the time of a load shutdown in the gas turbine apparatus; Fig. 6 is a diagram showing results of the operation of one example of a gas turbine apparatus; and Fig. 7 is a diagram showing other results of the operation of the gas turbine apparatus of Fig. 6. As shown in Fig. 1. to 3 the gas turbine 113 is provided with a gas turbine control system 1. The gas turbine control system 1 is a digital control system, and effects the optimum control at the time of start-up and stop of the gas turbine and during the operation of a load so as to enable an automatic operation of a gas turbine power generating unit. Control functions for this purpose includes an adjustment control function and a sequence control function. This gas turbine control system 1 further includes a protective device which gives an alarm when conditions undesirable for the operation of the gas turbine arise, and automatically stops the gas turbine apparatus. The adjustment control function cooperates with the sequence control function to adjust an amount of supply of fuel and so on in accordance with the operating condition of the gas turbine. In order to achieve this function, the gas turbine control system 1 includes a start-up control circuit 22, an acceleration limitation control circuit 20, a speed/load control circuit 24, a load limitation control circuit 26, an exhaust gas temperature control circuit 30, a fuel gas control circuit 36, a fuel pressure control circuit 38 and a compressor inlet guide vane control circuit. The start-up control circuit 22 serves to set values of fuel flow rate instructions for ignition, warming-up and acceleration at the start-up of the gas turbine. The acceleration limitation control circuit 20 serves to monitor the rotational speed 10 of the gas turbine at the start-up of the gas turbine, and effects a control so that the rate of acceleration of the turbine will not exceed a limit value. The speed/load control circuit 24 serves to control the operation of a governor in accordance with a speed in synchronism with the gas turbine system after the achievement of a rated speed and also in accordance with a permanent speed variation during the operation of a load. The adjustment of a governor setting circuit 18 at the synchronous speed operation is effected by the gas turbine control system. On the other hand, the output of the governor setting circuit 18 at the time of the operation of a load is either so adjusted that it is maximized within the limit of an output of the exhaust gas temperature control circuit 30, or is automatically adjusted in a load control means 16 so that an output value of a load setting circuit 14 manually set by the operator can be an output 12 of a generator. The load limitation control circuit 26 serves to effect a constant-output operation by load limitation setting during the operation of the load. The exhaust gas temperature control circuit 30 serves to effect a limit control of the temperature of the exhaust gas of the gas turbine at the start-up and during the operation of the load. At the start-up, this circuit 30 controls the fuel so that the temperature of the exhaust gas of the gas turbine detected by a thermocouple 66 will not exceed an allowable value, and also during the operation of the load, this circuit 30 limits the maximum output in accordance with the temperature of the exhaust gas of the gas turbine set by a temperature setting means 28 in accordance with an output of a pressure transducer 64 representing a pressure at an outlet of a combustor. In the temperature limit control, instead of the temperature of the inlet of the gas turbine, the temperature of the outlet of the gas turbine is measured, and a value, corrected by a discharge pressure of the compressor of the gas turbine, is used for this control. A minimum signal selection circuit 32 selects the least signal as a fuel control signal among the signals outputted respectively from the above five control circuits. The fuel gas control circuit 36 determines the degree of opening of fuel flow rate control valves in accordance with the fuel flow demand signal (FFD) 34, outputted from the minimum signal selection circuit 32, to control an amount of the fuel to be injected to the combustor 124. On the other hand, the fuel pressure control circuit 38 controls pressure control valves 46 and 52 so that outputs of pressure transducers 44 and 50, provided respectively before the fuel flow rate control valves 42 and 48, can be their respective predetermined values corresponding to the rotational speed the gas turbine. The compressor inlet guide vane control circuit serves to adjust an amount of the suction (intake) air so that the temperature of the exhaust gas of the gas turbine is made maximum within an allowable limit. At the start-up, in order to prevent pulsation of the air generating in the compressor of the gas turbine, the minimum opening degree is maintained by this circuit. The gas turbine apparatus is operated while effecting such various controls. At the time of a load shutdown, the fuel flow rate control valves 42 and 48 are abruptly closed in accordance with the output of the fuel control circuit 36. This will now be described with reference to Figs. 2 and 3. The compressor 101 feeds city gas or liquefied natural gas under pressure to the combustor 124 of the gas turbine. The compressor 101 comprises a low-pressure stage 121 and a high-pressure stage 122, and an intermediate cooler 105 is provided between the two stages, and the fuel, raised in temperature at the low-pressure stage, is cooled by this cooler, and then is fed to the high-pressure stage. An orifice 104, a suction valve 103 and a suction check valve 102, arranged in this order from the low-pressure stage 121, are connected at an upstream side of the low-pressure stage 121 through piping. On the other side, a discharge check valve 106 and a discharge tank 109, arranged in this order from the high-pressure stage 122, are connected to a downstream side of the high-pressure stage 122 through piping. At a downstream side of the discharge tank 109, fuel pressure control valves 46 and 52 and the fuel flow rate control valves 42 and 48, of the gas turbine 113 are connected in series respectively, through respective fuel pipe lines. The fuel pipes are, for example, a pipe having a diameter of 4 inches (101,6 mm). Although it is preferred to use a centrifugal compressor as the compressor, other turbo compressor may be used in the present invention. In the description given below, a centrifugal compressor is used as the compressor. A pipe (bypass pipe) which goes around the centrifugal compressor 101 (This flow will be hereinafter referred to as "bypass" for convenience) is connected between a point near to the outlet of the high-pressure stage 122 of the centrifugal compressor 101 and a point intermediate the suction check valve 102 and the orifice 104 connected to the low-pressure stage 121. A bypass valve 107 and a bypass cooler 108 are provided midway this bypass pipe. As the flow rate of the centrifugal compressor 101 is decreasing, the centrifugal compressor 101 is brought into a surging region, and finally causes a vibration phenomenon. Therefore, in order to prevent the surging of the centrifugal compressor 101, the rate of flow of the gas through the centrifugal compressor is increased, and part of the gas discharged from the centrifugal compressor is returned to the suction side by the provision of the bypass passage. Further, a sensor for detecting a pressure purposes is provided in the vicinity of the outlet of the high-pressure stage 122, and an output of this sensor is fed to a pressure indication regulator 140 by a transmitter 146. On the other hand, the flow rate of the fuel, measured by the orifice 104 provided upstream of the low-pressure stage 121, is fed to a fuel flow rate indication regulator 144 by a flow rate transmitter 142. In this embodiment, the relation between the fuel demand (FFD) of the gas turbine and the degree of opening of the bypass valve 107 is so determined that when the fuel flow demand (FFD) becomes small, the bypass valve 107 is automatically opened. A control system can be used in combination, in which a flow rate sucked by of the compressor is monitored by an orifice as in the conventional system, and when the flow rate reaches its lower limit value, the bypass valve 107 is opened. In this case, when either of the fuel flow demand (FFD) and the suction flow rate control command is inputted, the bypass valve 107 is opened. In either of these methods, a centrifugal compressor surge protection control system 70, including the pressure transmitter 146, the pressure indication regulator 140, the flow rate transmitter 142 and the flow rate indication regulator 144, operates, and as a result a centrifugal compressor bypass valve opening degree command 74 is given. Incidentally, in the event of an abrupt change of the load, the response to valve opening is too late with the conventional method of opening the bypass valve, and therefore the discharge pressure of the centrifugal compressor instantaneously increases, so that the rotational speed of the gas turbine exceeds the upper limit value. To prevent this, the use of the valve capable of achieving the shut-off instantaneously (about 0.1 second) involves a high cost when the fuel pipes with as large a diameter as 4 inches (101,6 mm) are used, and besides the apparatus is much increased in size. Therefore, the gas turbine fuel flow demand (FFD) 34 is inputted to the bypass valve of the centrifugal compressor, which feeds the fuel under pressure simultaneously when this fuel flow demand (FFD) is inputted to the fuel gas control circuit 36. Here, in each individual gas turbine apparatus, a predetermined relation is established between the fuel flow demand 34 and the degree of opening of the fuel gas bypass valve 107. More specifically, for example, as shown in Fig. 4, the bypass valve 107 is fully opened when the fuel flow demand is not less than 70%, and as the fuel flow demand is decreasing, the bypass valve 107 is gradually closed, and the degree of opening of the bypass valve is reduced to 40% when the fuel flow demand is 0%. This relation is stored in an associated memory of the gas turbine control system 1, and each time the fuel control signal is renewed, the stored value is read from the memory, so that a change in the load can be rapidly dealt with. In Fig. 5, when the fuel flow demand (FFD) 34 produces a fuel decrease demand, the degree of opening of the gas turbine fuel flow rate control valves 42, 48 decreases in accordance with the demand amount of FFD. Since the fuel decrease demand is fed, the gas turbine control system 1 commands the control device of the centrifugal compressor to reduce the amount of feed of the fuel under pressure. At this time, if the flow rate of the centrifugal compressor is merely reduced, the centrifugal compressor causes surging, and therefore the rate of flow of the gas through the centrifugal compressor needs to be above a predetermined level which will not cause surging. Therefore, the control device of the centrifugal compressor produces the valve opening command for the bypass valve of the centrifugal compressor. This command is produced generally simultaneously with the production of the fuel flow demand (FFD), and therefore the opening degree of the bypass valve of the centrifugal compressor produces less time lag relative to the demand as compared with the conventional system. The opening and closing operation of the bypass valve is a mechanical operation, and therefore it takes a longer time for the demand to be achieved as compared with other operations. When the suction flow rate valve 103, the bypass control valve 107 and the flow control valves 42, 48 are controlled as described above, the discharge pressure of the centrifugal compressor will not exceeds its upper limit value as shown at the bottom of Fig. 5, so that the surging can be prevented. Next, one example of the result of operating the gas turbine in accordance with this principle will now be described with reference to Figs. 6 and 7. As shown in Fig. 7, when the fuel flow rate control valves are fully closed from 37% to 0% in 2 seconds, the fuel gas flow rate increases from about 6,100 m 3 /h to about 7,100 m 3 /h . On the other hand, the pressure of the fuel gas at the outlet of the centrifugal compressor merely increases from about 25 kg/cm 2 to about 27.7 kg/cm 2 , so that the surging, which is an adverse effect for the centrifugal compressor, can be prevented from developing. The fact that the surging is not produced can be confirmed from the diagram indicating the discharge gas pressure. Therefore, there can be obtained the gas turbine apparatus of a high reliability. Moreover, by effecting the above control, a pressure fluctuation at the outlet of the centrifugal compressor can be reduced, and the volume of the discharge tank 109 provided for the purpose of absorbing the pressure fluctuation can be reduced.

Claims: A gas turbine apparatus comprising a gas turbine (113) with a turbine stage (123, 125), a combustor (124), a turbo compressor (101) for feeding city gas or liquefied natural gas under pressure to said combustor (124) and a gas turbine control device for controlling said gas turbine (113), which gas turbine control device comprises fuel flow rate control means (36, 42, 48) for controlling the rate of the flow of combustible gas, characterized in that said gas turbine (113) comprises a bypass passage extending from an outlet of said turbo compressor (101) to an inlet of said turbo compressor (101), through which bypass passage the combustible gas is able to flow from said outlet of said turbo compressor (101) to said inlet of said turbo compressor (101), and an opening/closing means (107) for opening and closing said bypass passage and said gas turbine control device comprises means for producing a signal (34) for driving said opening/closing means (107) and said fuel flow rate control means (36, 42, 48) in synchronism with each other. A gas turbine apparatus according to claim 1, characterized in that said gas turbine control device comprises a storage means which beforehand stores a relation between a value of said signal (34) produced by said signal (34) producing means and a degree of opening of said opening/closing means (107). A gas turbine apparatus according to claim 1, characterized in that said gas turbine control device comprises start-up control means (22), acceleration control means (20), speed/load control means (24), load limitation means (26), and exhaust gas temperature control means (30), and said means for producing the signal (34) for driving said opening/closing means (107) and said fuel flow rate control means (36, 42, 48) in synchronism with each other is a selection means (32) for selecting the least one among control signals outputted respectively from the above five control means (20, 22, 24, 26, 30) and for outputting said least signal as said signal (34) for driving said opening/closing means (107) and said fuel flow rate control means (36, 42, 48) in synchronism with each other. A method of operating a gas turbine apparatus using as a fuel liquefied natural gas or city gas fed under pressure by a turbo compressor (101), characterized by the steps of producing a fuel control signal (34) in accordance with at least one of signals outputted respectively from start-up control means (22), acceleration control means (20), speed/load control means (24), load limitation means (26), and exhaust gas temperature control means (30) provided in a gas turbine control device, and controlling a rate of flow of the fuel into said gas turbine apparatus as well as a rate of flow of the fuel around said turbo compressor (101) by said gas turbine control device in accordance with said fuel control signal (34). A method according to claim 4, characterized in that said fuel control signal (34) is outputted to fuel flow rate control means (36, 42, 48) of said gas turbine apparatus and flow-around flow rate control means (107) of said turbo compressor (101) in a synchronous manner.

Inventors: HONDA YOSHITSUGU, NEGISHI TADANOBU, YAMADA HIROSHI, HONDA, YOSHITSUGU, NEGISHI, TADANOBU, YAMADA, HIROSHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1994
Main classification: F02C3
IPC codes: F02C9, F04D27, F02C3